Conventional toggle clamps comprise at least one clamping arm that is pivotally connected through links to a base support. It is well known that, as the links toggle, a point is reached and passed at which the compressive and tension stresses in the links and clamp arm are theoretically infinitely large.
The internal stress in the links and the clamping arm is relieved to some extent by their inherent (albeit slight) elasticity, which can save the parts from fracturing. Still, one undesirable consequence of the great unpredictable internal stress occurring at the toggle point is that the pins connecting the links and the pin on which the clamp arm swings are subject to a powerful shearing force. Since there is no motion between the pivot points and the links at the toggle point, the high shearing or bearing stress causes rapid wear of the pins and/or in the links, which are journaled on the pins. If the size of the article being clamped varies by as little as 0.015 inch (0.38 mm) the clamping force can vary by 25%-50%.
If the article size exceeds specifications, internal stress of the clamp parts is even greater. If the article size is under specification, the article may slip in the clamp which can result in damage to property or injury to a person in an industrial setting.
Furthermore, clamps or grippers without mechanical locking can release parts during a pressure loss or pressure drop. The failure at an unclamped condition can occur when pressure decreases after locking. When the pressure decreases, the cylinder has less available force to open. This problem is aggravated by mechanisms designed to clamp on the advance stroke of the cylinder.
A number of clamp manufacturers have moved to so-called "Wedge Locking" designs in order to avoid the toggle unlocking problem. Wedge Locking clamps are designed to stop before reaching a locked position and therefore are not true locking mechanisms. Under severe vibration, wedge clamps can lose mechanical locking.
Furthermore, current enclosed clamp designs have a cantilevered shaft protruding from the side of the clamp where the arm is attached. This type of loading causes bending, torsion, and shear loading, severely reducing the load capacity of the clamp.
These shortcomings, taken alone or in combination, call for improved clamping and gripping devices.